An array of light emitting diodes, which is formed by a plurality of p-n junction light emitting diodes or p-i-n junction light emitting diodes closely positioned on a substrate, is advantageous in that it enables image information to be processed relatively easily by electrically controlling each of the diodes, and also has several applications.
In response to the recent increase in not only the quantity of information, but also the quality of information to be handled, which now includes in addition to written information, image information such as drawings, photographs, and so on, a printer has become necessary which has a higher dot density and faster printing speed.
As a response to these requirements, a light emitting diode printer was developed which uses an array of light emitting diodes as a light source. The printer has an optical write head including an array of solid state high density light emitting diodes positioned along a photosensitive drum. Light from the head is focused on a photosensitive drum through an array of converging rod lenses. By this arrangement, an electrostatic latent image is formed such that the light emitting diodes are selectively switched on and off in accordance with an image signal.
The optical write head comprises a plurality of array chips, such as thirty five chips, each of which normally has 64 to 256 light emitting diodes integrated with a predetermined density, e.g., 16 elements/mm.
Thus, in printers using light emitting diodes, as each of the light emitting diodes is electrically controlled, no mechanical driving device is needed. That is, no space for light deflection is required and it has no moving part, but it may use a simple equimagnification array lens in an optical system. Therefore, the light emitting diode printers are also known as non-impact optical printers, and can theoretically be constructed to be very small and highly reliable compared with laser printers which require mechanical arrangements such as rotatable polygon mirrors for light beam scanning and a corresponding intricate optical system.
FIG. 1 shows a sectional view of an example of a prior art light emitting diode array 200 applicable to a light emitting diode printer, and its arrangement and a manufacturing process will be explained with reference to this figure. The array 200 comprises a substrate 210 of n-type conductivity gallium arsenide (GaAs) having a pair of opposed surfaces 209 and 211. On the surface 211 of the substrate 200 is a layer 212 of n-type conductivity gallium arsenide phosphide (GaAsP). Within the GaAsP layer 212 are spaced apart regions 216 which are doped with zinc so as to be of p-type conductivity. On the surface of the GaAsP layer 212 is a layer 214 of an insulating material, such as silicon nitride (SiN), having openings 215 therethrough over the p-type regions 216. A p-electrode 218 is on each of the p-type regions 216 and an n-electrode 220 is on the surface 209 of the substrate 210. The n-electrode 220 is alloyed with the substrate 210 to form an alloyed region 224 along its interface with the substrate 210. A protection layer 222 of an insulating material, such as silicon oxide (SiO.sub.x) covers the silicon nitride layer 214, the p-electrodes 218 and any exposed portions of the p-type regions 216. In order to simplify the description, only two light emitting diodes 226 are shown in FIG. 1, each of which is formed by a p-type region 216 and the n-type GaAsP layer 212.
A process for manufacturing the array 200 of light emitting diodes 226 shown in FIG. 1 will now be explained. On the n-type GaAs substrate 210, the n-type GaAsP layer 212 is deposited by means of a VPE (Vapor Phase Epitaxy) method. Then, on the upper surface of the n-type GaAsP layer 212, spared apart p-type regions 216 are formed by means of zinc (Zn) diffusion using the SiN film 214 as a mask. Interfaces thus formed between the n-type GaAsP layer 212 and the p-type regions 216 are p-n junctions 228 to provide light emitting regions.
Partly on the upper surface of each of the p-type regions 216 and on the lower surface 209 of the n-type GaAs substrate 210, metal layers are respectively deposited to form predetermined patterns by means of vapor deposition etc., and then the whole wafer is heated to form alloys from the metals. In this way, the p-electrodes 218 and the n-electrode 220 are formed. Furthermore, the silicon oxide film 222 is also applied on the upper surface as a non-reflective coating. The portion of the silicon oxide film 222 which is located remote from the light emitting diodes is removed to form bonding pads (not shown) of the p-electrodes 218.
In each of the light emitting diodes 226 in such an array 200 as mentioned above, actual light emission depends on an amount of electric current flowing through the p-n junction region 228. Also, since a current distribution in the n-type GaAs substrate 210 depends on an electrical field induced therein, and an electrical field is strongest at the regions located immediately below the p-electrodes 218, the amount of current flowing through these regions is highest.
However, even if each diode emits a light at the region located immediately below each of the p-electrodes 218, the upper portion of such a light emitting region is covered with the p-electrode 218, which results in not only a low efficiency in taking light from that region to the outside, but also a wasteful dissipation of heat generated by the current injected from each of the p-electrodes 218.